Particle blasting systems have been around for several decades. Typically, particles, also known as blast media, is fed into a transport gas flow and are transported as entrained particles to a blast nozzle, from which the particles exit, being directed toward a workpiece or other target. It is not unknown for the particles to clump or stick together, impeding the delivery of particles into the transport gas flow.
Such compaction and agglomeration of particles is particularly a problem when the blast media is cryogenic particles, such as in carbon dioxide blasting. Although still a relatively young industry, carbon dioxide blasting systems are well known in the industry, and along with various associated component parts, are shown in U.S. Pat. Nos. 4,744,181, 4,843,770, 4,947,592, 5,050,805, 5,018,667, 5,109,636, 5,188,151, 5,301,509, 5,571,335, 5,301,509, 5,473,903, 5,660,580 and 5,795,214, all of which are incorporated herein by reference. Although the present invention will be described herein in connection with a particle feeder for use with carbon dioxide blasting, it will be understood that the present invention is not limited in use or application to carbon dioxide blasting. The teachings of the present invention may be used in application in which there can be compaction or agglomeration of any type of particle blast media.
Generally, the blast media particles, such as carbon dioxide particles, are transported from a hopper, which holds the supply of particles, into a transport gas. The particles may be introduced into the transport gas by venturi or other vacuum effect, or by a feeder. Various feeder designs exist, functioning to transport the particles from the hopper exit into the transport gas, such as by the radial transport feeder shown in U.S. Pat. No. 4,947,592. Hoppers may receive particles from any source, such as a pelletizer that is part of the blast system, or a source separate from the blast system and loaded into the hopper.
Prior attempts in the art to promote the flow of particles, and in particular cryogenic particles, to and through the exit of a hopper or other storage/feeder structure include the use of vibrators or thumpers which act on the walls of the hopper and the use of vertically oriented rotating augers and stirrers in or adjacent the hopper exit to mechanically advance the particles. Typically hoppers have been fairly rigidly connected to the blast system frame, which is now recognized to be a significant impediment to transferring sufficient energy to the hopper walls to effect the flow of particles. In such designs, a significant portion of the energy transferred to the hopper was also transferred through the hopper to the blast system frame. The energy that went to the frame produced undesirable results, manifested as noise, vibration and movement of the entire system, fatigue and stress in the hopper and frame, as well as the consumption of extra energy.
The desired higher total energy was difficult to achieve with thumpers, in which reciprocating plungers/strikers repetitively strike the hopper, as the size of the movable mass was a limiting factor. Each impact of a large mass against a hopper could undesirably cause the entire system to jump. Thus, the required level of energy was achieved though high frequency/low mass vibrators. High frequency, however, tends to compact the particles, impeding the flow. Vertical hopper walls compounded the compaction problem present with high frequency energy, forcing hopper walls away from vertical walls to inclined walls. However, hoppers with inclined walls have less internal capacity than hoppers with vertical walls.
With cryogenic particles, even when they are moved toward the exit of the hopper, they may easily bridge the exit, or form agglomerated clumps too large to be ingested by the feeder mechanism, slowing or blocking particle flow.
Thus, there is a need in the art for particle blast system that has improved, reliable particle flow from the hopper to the hopper exit and on to the transport gas.